1. Field of Invention
This invention relates to an X-ray detector and method of making same; and, more particularly, to an X-ray detector for use in an X-ray computerized tomograph system, often referred to as "X-ray CT".
2. Description of the Prior Art
X-ray computerized tomograph systems generally have an X-ray tube for irradiating X-rays toward an object under examination and an X-ray detector consisting of a plurality of partitioned detection chambers for detecting X-rays transmitted through the object. The detected data are processed by a computer to obtain tomograms of the object.
A conventional X-ray detector for use in X-ray CT systems is now described with reference to FIG. 1, to show the principles of operation thereof. FIG. 1 shows a conventional X-ray detector in schematic form with its casing partially cut away for sake of clarity. The detector has a casing 1 filled with a gas 2 under high pressure, for detecting X-rays. The kind of gas 2 is selected from known rare gases having large atomic numbers, such as, for example, xenon, krypton, argon, etc. The pressure and other factors, outside of the invention, as described herein, are determined in accordance with known techniques. The front portion 3 of casing 1 is made of a material which is substantially transpararent to X-rays, such as plastics or aluminum. Gas 2 filling casing 1 is substantially opaque to radiations in the range of X-ray frequencies and front portion 3 is substantially transparent to such radiations. A beam of X-rays to be detected is indicated by arrow 5 and enters front portion 3 of casing 1, in the direction indicated by arrow 5, as shown in FIG. 1. Flat signal electrode plates 6 and flat bias electrode plates 7 are disposed alternately in substantially parallel relation to one another at substantially regular intervals, and are oriented substantially parallel to the incident direction of X-rays 5, as depicted. Thus, spaces formed by adjacent bias electrode plates and signal electrode plates define separate chambers for detecting the X-rays passing at different positions. Bias electrode plates 7 are connected to a bus line through which a DC voltage V.sub.1 (see for example FIG. 6) is applied across signal electrode plates 6 and their respective neighboring electrode plates 7.
In the operation of the detector just described, when X-rays enter the different detection chambers, after penetrating the front portion 3, the X-rays interact with the atoms of gas 2, to produce electron-ion pairs, which flow along the electric field, thus to produce electric current across signal electrode plates 6 and their neighboring bias electrode plates 7. Since the value of the generated current corresponds to the incident X-ray energy, measurement of each current flowing into lead wires connected to the respective signal electrode plates 6, allows the determination of the X-ray energy for each respective detection chamber, which represents the different portions through which the X-rays passed through the object.
Advantageously, X-ray tomographs can produce sectional images, that would be obtained if the object undergoing medical examination were cut into slices, to assist the examination. Consequently, it is desired that the resultant sectional images provide a high resolution. That is, images distinctly depicting even minute portions within the human organs are desirable. The degree of such resolution is used as one of the major factors in judging the performance of X-ray tomograph systems.
The resolution of images depends strongly on the construction and characteristics of the X-ray detector used. Improvement in resolution can be achieved, it is believed, by more densely packing the signal and bias plates in the detector. Heretofore, various constraints have imposed limitations to high density packing of the electrode plates, as hereinafter discussed with reference to FIGS. 2-7.
Referring to FIG. 2, there are shown signal electrode plates 6 and bias electrode plates 7. These plates are disposed alternately at substantially regular intervals in a conventional manner. This drawing figure is an elevational view taken in the incident direction of the X-rays shown in FIG. 1. Plates 6 and 7 are supported by supporting structure 9. FIG. 3 is a fragmentary enlarged view of the detector of FIGS. 1 and 2, and shows the manner in which plates 6 and 7 are supported by supporting structure 9. The structures 9 are provided with horizontally disposed grooves into which plates 6 and 7 are inserted at their upper and lower edges, and securely fixed to the supporting structure 9, with, for example, adhesive.
Since the signal and bias electrode plates 6 and 7 are required to be insulated from each other, the supporting structure 9 is usually made of an insulating material, such as ceramic. However, such material tends to crack easily, and hence, is not well suited for fine machining. Accordingly, as the intervals of the grooves formed in supporting structure 9 are made closer, the structure tends to crack more easily. Thus, a certain practical limit is imposed on the closeness of the grooves, and hence the resolution of the tomograms. This practical limit for prior arrangement leaves much to be desired in terms of resolution of the obtained tomograms.
Moreover, the prior art is further deficient in that signal electrode plates 6 are required to have lead wires attached thereto, in order to obtain electric currents proportional to the respective X-ray energies incident on the respective detection chambers. Also, bias electrode plates 7 are required to be connected to a high voltage power source, for example, of about 500 volts. For these purposes, the prior art has used a connecting structure, such as shown in FIG. 4, wherein lead wires l.sub.s and l.sub.c and bus line L, which is maintained at a high voltage, are shown. Lead wires l.sub.s and l.sub.c are connected to electrode plates 6 and 7, respectively, by soldering, bonding or the like. Accordingly, connecting points P protrude perpendicularly from the sides of the plates, thus preventing close arrangement of the adjacent plates 6 and 7.
Referring next to FIG. 5, a signal electrode plate 6, provided with conventional means for heightening the resolution of images, is shown. The plates 6 comprise a flat plate 61 made of an insulating material, and electrodes 62 on opposite sides of plate 61, and made of electrically conductive material. Electrodes 62 are electrically connected to their respective lead wires l.sub.s via metallic clips 63, as depicted.
A conventional x-ray detector, using signal electrode plates 6, such as depicted in FIG. 5, has an electrode arrangement, such as shown in FIG. 6, wherein parts described already in connection with FIGS. 1-5, and denoted with like reference numerals, are not again described for sake of simplicity and clarity of description. In FIG. 6, using the plate arrangement of FIG. 5, one detection chamber 10, for one channel, has a width d. On the other hand, as shown in FIG. 3, if each signal electrode plate 6 is made simply of a metallic plate, such as in FIGS. 1,2,3,4, detection chambers corresponding to one channel, have width D. As can be seen from these two FIGS. 6 and 3, the relationship d.apprxeq.D/2 holds. Thus, the arrangement of FIG. 6 can be said to be an improvement in resolution, to the arrangement of FIG. 3. However, such improvement per se is still not sufficient. The X-ray detector of FIG. 6 still has a disadvantage which prevents the obtaining of good distinct and clear tomograms. This disadvantage will now be described.
Referring again to FIG. 6, a large capacitance C.sub.1 (see FIG. 7) is produced between each adjacent channel, for example, a and b, because of the interposing of insulator 61. Electric current produced across two channels is applied to an output circuit, such as shown in FIG. 7, wherein current is converted into voltage by way of an integrator circuit (not shown) to measure the value of the current. The circuit of FIG. 7 includes amplifiers U.sub.1 and U.sub.2. The aforementioned capacitance C.sub.1 causes interference between the two channels, thus, hindering production of distinct and clear tomograms of the different channels.
Thus, the prior art X-ray computerized tomograph systems are lacking in X-ray detectors, which can produce high resolution tomographs, consistent with acceptable costs and operating characteristics.